// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef NET_DNS_DNS_RESPONSE_H_
#define NET_DNS_DNS_RESPONSE_H_

#include <stddef.h>
#include <stdint.h>

#include <string>

#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/strings/string_piece.h"
#include "base/time/time.h"
#include "net/base/net_export.h"

namespace net {

class AddressList;
class DnsQuery;
class IOBufferWithSize;

namespace dns_protocol {
    struct Header;
}

// Parsed resource record.
struct NET_EXPORT_PRIVATE DnsResourceRecord {
    DnsResourceRecord();
    ~DnsResourceRecord();

    std::string name; // in dotted form
    uint16_t type;
    uint16_t klass;
    uint32_t ttl;
    base::StringPiece rdata; // points to the original response buffer
};

// Iterator to walk over resource records of the DNS response packet.
class NET_EXPORT_PRIVATE DnsRecordParser {
public:
    // Construct an uninitialized iterator.
    DnsRecordParser();

    // Construct an iterator to process the |packet| of given |length|.
    // |offset| points to the beginning of the answer section.
    DnsRecordParser(const void* packet, size_t length, size_t offset);

    // Returns |true| if initialized.
    bool IsValid() const { return packet_ != NULL; }

    // Returns |true| if no more bytes remain in the packet.
    bool AtEnd() const { return cur_ == packet_ + length_; }

    // Returns current offset into the packet.
    size_t GetOffset() const { return cur_ - packet_; }

    // Parses a (possibly compressed) DNS name from the packet starting at
    // |pos|. Stores output (even partial) in |out| unless |out| is NULL. |out|
    // is stored in the dotted form, e.g., "example.com". Returns number of bytes
    // consumed or 0 on failure.
    // This is exposed to allow parsing compressed names within RRDATA for TYPEs
    // such as NS, CNAME, PTR, MX, SOA.
    // See RFC 1035 section 4.1.4.
    unsigned ReadName(const void* pos, std::string* out) const;

    // Parses the next resource record into |record|. Returns true if succeeded.
    bool ReadRecord(DnsResourceRecord* record);

    // Skip a question section, returns true if succeeded.
    bool SkipQuestion();

private:
    const char* packet_;
    size_t length_;
    // Current offset within the packet.
    const char* cur_;
};

// Buffer-holder for the DNS response allowing easy access to the header fields
// and resource records. After reading into |io_buffer| must call InitParse to
// position the RR parser.
class NET_EXPORT_PRIVATE DnsResponse {
public:
    // Possible results from ParseToAddressList.
    enum Result {
        DNS_PARSE_OK = 0,
        DNS_MALFORMED_RESPONSE, // DnsRecordParser failed before the end of
        // packet.
        DNS_MALFORMED_CNAME, // Could not parse CNAME out of RRDATA.
        DNS_NAME_MISMATCH, // Got an address but no ordered chain of CNAMEs
        // leads there.
        DNS_SIZE_MISMATCH, // Got an address but size does not match.
        DNS_CNAME_AFTER_ADDRESS, // Found CNAME after an address record.
        DNS_ADDRESS_TTL_MISMATCH, // OBSOLETE. No longer used.
        DNS_NO_ADDRESSES, // OBSOLETE. No longer used.
        // Only add new values here.
        DNS_PARSE_RESULT_MAX, // Bounding value for histograms.
    };

    // Constructs a response buffer large enough to store one byte more than
    // largest possible response, to detect malformed responses.
    DnsResponse();

    // Constructs a response buffer of given length. Used for TCP transactions.
    explicit DnsResponse(size_t length);

    // Constructs a response from |data|. Used for testing purposes only!
    DnsResponse(const void* data, size_t length, size_t answer_offset);

    ~DnsResponse();

    // Internal buffer accessor into which actual bytes of response will be
    // read.
    IOBufferWithSize* io_buffer() { return io_buffer_.get(); }

    // Assuming the internal buffer holds |nbytes| bytes, returns true iff the
    // packet matches the |query| id and question.
    bool InitParse(int nbytes, const DnsQuery& query);

    // Assuming the internal buffer holds |nbytes| bytes, initialize the parser
    // without matching it against an existing query.
    bool InitParseWithoutQuery(int nbytes);

    // Returns true if response is valid, that is, after successful InitParse.
    bool IsValid() const;

    // All of the methods below are valid only if the response is valid.

    // Accessors for the header.
    uint16_t flags() const; // excluding rcode
    uint8_t rcode() const;

    unsigned answer_count() const;
    unsigned additional_answer_count() const;

    // Accessors to the question. The qname is unparsed.
    base::StringPiece qname() const;
    uint16_t qtype() const;

    // Returns qname in dotted format.
    std::string GetDottedName() const;

    // Returns an iterator to the resource records in the answer section.
    // The iterator is valid only in the scope of the DnsResponse.
    // This operation is idempotent.
    DnsRecordParser Parser() const;

    // Extracts an AddressList from this response. Returns SUCCESS if succeeded.
    // Otherwise returns a detailed error number.
    Result ParseToAddressList(AddressList* addr_list, base::TimeDelta* ttl) const;

private:
    // Convenience for header access.
    const dns_protocol::Header* header() const;

    // Buffer into which response bytes are read.
    scoped_refptr<IOBufferWithSize> io_buffer_;

    // Iterator constructed after InitParse positioned at the answer section.
    // It is never updated afterwards, so can be used in accessors.
    DnsRecordParser parser_;

    DISALLOW_COPY_AND_ASSIGN(DnsResponse);
};

} // namespace net

#endif // NET_DNS_DNS_RESPONSE_H_
